The present invention relates to a magneto-optical recording medium, such as magneto-optical disk, magneto-optical tape, and magneto-optical card, and a method of reading the same, and more particularly to a magneto-optical recording medium that can be reproduced at magnetically induced super-resolution.
In recent years, magneto-optical disks have been attracting much attention as external storage media for computers. The magneto-optical disk uses an external magnetic field and a laser beam to form record marks of submicron size on the medium, and can achieve a drastic increase in storage capacity as compared with other external recording media such as flexible disks and hard disks.
To enable the recording of a tremendous amount of data such as moving pictures, a further increase in the storage capacity of magneto-optical disks is demanded. To increase the recording density, it is needed to form more record marks on the medium, that is, to define the length of record mark shorter than the spot diameter of Laser beam and to narrow the interval of record marks. Forming of such fine record marks is relatively easy, but, there is a limit in the length of record marks that can be read due to restrictions of the wavelength (.lambda.) of the laser beam to be emitted and the numerical aperture (NA) of the objective lens.
Accordingly, various magnetically induced super-resolution (MSR) readout methods for reading record marks smaller than the laser beam diameter have been proposed (Japanese Patent Application Laid-Open Nos. 1-143041 (1989), 3-93056 (1991), 3-93058 (1991), 4-271039 (1992), and 5-12731 (1993)). In all these conventional methods, a magneto-optical disk laminating plural magnetic layers including a recording layer and a readout layer is rotated, a readout laser beam is emitted to cause a temperature distribution in the peripheral direction of the magneto-optical disk, and record marks smaller than the spot diameter are read by making use of this temperature distribution. That is, in a certain temperature region within a spot of readout laser beam, the readout layer has a direction of magnetization so as to mask the record mark, and in other region, on the other hand, the direction of magnetization of the recording layer is transferred on the readout layer and is read out.
In these conventional methods, the record mark can be read out from a region smaller than the spot diameter of the readout laser beam, which substantially brings about the same resolution as when reading out by a light spot smaller than the spot diameter of the readout laser beam. These conventional methods, however, had the following problems. First, in the method of reading out record marks from the low-temperature region in the spot, although the resolution is excellent in the peripheral direction, the crosstalk by adjacent tracks is significant, or in the method of reading out record marks from the high-temperature region in the spot, although the crosstalk is decreased, a large initializing magnet is needed for initializing the readout layer, and the apparatus is not reduced in size, and further in the method of reading out record marks from a region changed in the direction of magnetization of the readout layer from the in-plane direction to perpendicular direction due to temperature distribution, although it is possible to read out without using a large initializing magnet, the transferred region in the spot is wide, and high readout output is not obtained.
Accordingly, the present applicant proposed an MSR readout method capable of solving these problems (Japanese Patent Application Laid-Open No. 7-244877 (1995)). FIG. 1 is a diagram showing the film composition of the conventional magneto-optical disk capable of reading out at MSR proposed by the present applicant and the direction of magnetization in readout. FIG. 2 is a diagram showing the state of magnetization when erasing this magneto-optical disk, and FIG. 3 is a diagram showing the state of magnetization when recording.
As shown in FIG. 1, a magneto-optical disk 20 is formed by laminating a base layer (not shown) made of SiN, a readout layer 21, an intermediate layer 22, a recording layer 23 respectively made of rare-earth transition-metal amorphous alloy, and a protective layer (not shown) made of SiN sequentially on a polycarbonate resin substrate (not shown). The readout layer 21 is transition-metal magnetization dominant, and has an easy axis of magnetization in the perpendicular direction, that is, the lamination direction. The intermediate layer 22 is rare-earth magnetization dominant, and has an easy axis of magnetization in the in-plane direction at room temperature (10 to 35.degree. C.), and over a predetermined temperature higher than room temperature, the easy axis of magnetization is changed from the in-plane direction to the perpendicular direction. The recording layer 23 is transition-metal magnetization dominant, and has an easy axis of magnetization in the perpendicular direction. Supposing the Curie temperatures of the readout layer 21, intermediate layer 22 and recording layer 23 to be respectively Tc1, Tc2 and Tc3, the relation of Tc2&lt;Tc1, Tc2&lt;Tc1) is satisfied. Supposing the coercive forces of the readout layer 21 and recording layer 23 at room temperature to be respectively Hc1 and Hc3, the relation of Hc3&gt;Hc1 is satisfied.
When erasing record marks on the magnet-optical disk 20, as shown in FIG. 2, while applying an downward erasing magnetic field, a erasing laser beam is emitted. At this time, the region irradiated with the laser beam is heated at curie temperature Tc3 or more, so that the direction of magnetization of the recording layer 23 is aligned in the same downward direction as the erasing magnetic field. A region away from the erasing laser beam is cooled to room temperature. At room temperature, the intermediate layer 22 is an in-plane magnetized film as mentioned above, and the readout layer 21 and the intermediate layer 22 are not coupled magnetically. Therefore, the direction of magnetization of the readout layer 21 is aligned in the same downward direction as the erasing magnetic field. In the magneto-optical disk 20, meanwhile, the erasing direction is expressed downward, the recording direction, upward reverse to the erasing direction.
When recording record marks on the magneto-optical disk 20, as shown in FIG. 3, while applying an upward recording magnetic field, a recording laser beam is emitted. This recording method is realized by two manners, the light modulation recording and magnetic field modulation recording. The light modulation recording is a method of irradiating by modulating so that the intensity of laser beam may correspond to the information while always applying an upward recording magnetic field, and only the region irradiated with laser beam of high intensity has the same upward direction of magnetization as the recording magnetic field, so that record marks are formed therein. On the other hand, the magnetic field modulation recording is a method of applying by modulating the direction of magnetic field up and down so as to correspond to the information while always emitting recording laser beam, and the direction of magnetization in the region irradiated with the laser beam is aligned in the direction of the applied magnetic field. In making use of magnetic field modulation recording, when recording information from erased state, the direction of magnetization is inverted upward in the region applied with a magnetic field reverse to the erasing magnetic field, and record marks are formed therein.
A region away from the recording laser beam is cooled to room temperature. At room temperature, the intermediate layer 22 is an in-plane magnetized film as mentioned above, and the readout layer 21 and recording layer 23 are not coupled magnetically. Therefore, the direction of magnetization of the readout layer 21 is aligned in the direction of magnetization by applying a small magnetic field, and it is not necessary to use a large initializing magnet.
The state of magnetization in readout of thus recorded magneto-optical disk 20 is explained by reference to FIG. 1. A readout laser beam is emitted to the magneto-optical disk 20, and a downward readout magnetic field is applied to the irradiated region. In a low-temperature region ahead of the laser beam (a region lower than substantially 100.degree. C.), the exchange coupled force between the intermediate layer 22 and recording layer 23 is weak, and the magnetization of the intermediate layer 22 is aligned in the direction of readout magnetic field, that is, in the downward direction. By the exchange coupled force between the intermediate layer 22 and readout layer 21, the direction of magnetization of the readout layer 21 is aligned in the upward direction, and acts to mask the direction of magnetization of the recording layer 23 (front mask). A high-temperature region (a region higher than substantially 180.degree. C.) is a region beyond the Curie temperature of the intermediate region 22, and the exchange coupled force between the intermediate layer 22 and readout layer 21 is cut off. As a result, the direction of magnetization of the readout layer 21 is aligned in the direction of readout magnetic field, and acts to mask the direction of magnetization of the recording layer 23 (rear mask). In an intermediate-temperature region between the low-temperature region and the high-temperature region (a region of substantially 100.degree. C. to substantially 180.degree. C.), the direction of magnetization of the recording layer 23 is transferred onto the readout layer 21 by the exchange coupled force between the recording layer 23 and readout layer 21 through the intermediate layer 22.
Therefore, when a magneto-optical output is detected, since the mask is formed in the region of low temperature and region of high temperature in the laser spot S, the magneto-optical signal is not read out from the regions, so that the magneto-optical signal is read out only from the intermediate-temperature region.
Thus, according to the magneto-optical disk proposed by the present applicant, the MSR readout is enabled without using large initializing magnet, and a partial area of high-temperature region (an intermediate-temperature region) is an aperture, and a high readout output is obtained, and moreover since the adjacent tracks are lower-temperature regions than the intermediate-temperature region, so that the signal is not read out from the adjacent tracks, and therefore the crosstalk is low.
However, when varying the direction of magnetic field applied at the time of readout of thus constituted magneto-optical disk 20, that is, when the same upward readout magnetic field as the recording direction is applied in this magneto-optical disk 20, forming of front mask tends to be difficult. This is because the control of exchange coupled force is difficult at low temperature between the intermediate layer 22 and recording layer 23 in the prior art. When a readout magnetic field in the recording direction is applied, the direction of magnetization of the intermediate region 22 is not aligned with the readout magnetic field in low-temperature region, and forming of front mask is hence imperfect, and the readout characteristic deteriorates.